1. Field of the Invention
The invention relates generally to semiconductor and media fabrication equipment and more specifically to vacuum-insulated refrigerant line for allowing vacuum chamber systems with water-vapor cryocoil compressors to be located outside a device-fabrication cleanroom and to low heat loss interconnecting pipes that make such remote location possible.
2. Description of the Prior Art
Semiconductor and media devices are fabricated in "cleanrooms" that have high construction costs per square foot. It is therefore worthwhile and desirable to minimize the number of processing equipment devices designed to be inside this expensive space, as well as to miniaturize the devices that are absolutely necessary.
Vacuum chambers are very commonly used in thin-film storage media, flat panel display and semiconductor device fabrication cleanrooms. Much of the actual fabrication occurs while such devices are in a vacuum. Roughing, or mechanical, vacuum pumps can do a large part of the evacuation of the vacuum chambers, but the very high vacuums required often demand the use of more exotic pumps as well, e.g., cryopumps.
Water vapor can be removed from a pumped-down vacuum chamber by a cryopump to help improve the vacuum. For example, Polycold Systems, Inc. (San Rafael, Calif.) uses so-called "Meissner" cryocoils inside vacuum chambers to precipitate water vapor out of the internal atmosphere by condensation. The condensed water molecules therefore no longer contribute to the gas volume inside the chamber.
A refrigerant, such as FREON or ammonia, is compressed and pumped through a discharge line by a compressor to the cryocoil. It absorbs heat, and the heated refrigerant is returned in a suction line back to the compressor through an evaporator. In semiconductor cleanroom applications, the compressor alone can consume several square feet of floor space.
In order to improve efficiency, the suction and discharge pipe connections between a cryopump compressor and the cryocoil are usually foam-insulated and paired in a single covering. Pipes with simple foam-insulated wrappings can have heat leakages of fifteen to twenty-five BTUs per foot per hour. Pipes with rigid, high quality insulation can reduce this to about ten to twelve BTUs per foot per hour. Some excess heat leakage can be compensated for by using a larger compressor-evaporator, but this alone is expensive. Larger, more powerful units also require more of the expensive cleanroom floor-space. The maximum pipe runs in prior art equipment have been limited to just a few yards. A typical Polycold system uses eight foot pipe runs. Beyond fifteen feet, the usual solution is to install larger compressors and more expensive rigid-foam insulation.
The choices of materials and methods available to construct such piping between the compressor-evaporator and cryocoil is severely limited. The outer covering and the installation techniques must be "clean". They must not contribute significant contamination particles to the atmosphere of the cleanroom during construction or use. The outer covering, if it is to contain a high-quality insulator such as a vacuum, must be rigid enough to not collapse under the vacuum, and yet flexible enough to allow routing of the pipe during installation. The discharge and suction lines themselves also need to be very resistant to bursting, as the internal operating pressures can go as high as several hundred pounds-per-square-inch-gauge (PSIG).